Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or drawing an image on paper, film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure from a piezoelectric element, a system of jetting out a liquid droplet by generating a bubble in the ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The inkjet recording ink used therefor includes an aqueous ink, an oily ink and a solid (fusion-type) ink.
Among these inks, an aqueous ink is relatively superior to oily ink or solid (fusion-type) ink in view of possibility of satisfying all of production, handleability, odor, safety and the like and therefore, is predominating as the inkjet recording ink at present.
The coloring material used in such an inkjet recording ink is required to have high solubility in a solvent (ink medium), enable high-density recording, provide good color hue, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing to an image-receiving material and less bleeding, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. However, although various dyes and pigments for use in inkjet recording have been already proposed and are actually used, a coloring material satisfying all of the requirements described above is not yet found out at present.
Conventionally well-known dyes and pigments such as coloring material having a color index (C.I.) number can hardly satisfy all of those various properties required of the inkjet recording ink.
As for the dye capable of improving the fastness, azo dyes derived from an aromatic amine and a 5-membered heterocyclic amine have been proposed in Patent Document 1. However, these dyes have a problem of bad color reproducibility due to undesirable color hue present in the yellow and cyan regions. Patent Documents 2 and 3 are disclosing an inkjet recording ink with an attempt to enhance the light fastness. However, in use as a water-soluble ink, the coloring materials used in these patent publications are insufficient in the solubility in water. Also, when used as a water-soluble ink for inkjet recording, there arises a problem in the fastness to humidity and heat.
In order to solve these problems, compounds and ink compositions described in Patent Document 4 have been proposed. Furthermore, an inkjet recording ink using a pyrazolylaniline azo dye for improving the color hue and light fastness is described (Patent Document 5). However, these inkjet recording inks are still insufficient in the fastness of image output.
In addition to the image preservability, more reduction in the blurring of image is recently demanded accompanying requirement for a highly refined inkjet image. In preparing an ink, when only water is used, the penetrability into a medium is bad and the image is not fixed in many cases. Also, the liquid properties necessary for hitting as an ink are often not satisfied. For solving these problems, a technique of using a water-miscible high boiling point organic solvent as auxiliary solvent and using a nonionic surfactant is commonly employed in this field. However, the formulation of preparing an ink containing an auxiliary solvent and a surfactant for this purpose has a problem that blurring of the image drawn increases particularly under high-humidity conditions.
Out of those various properties required of the inkjet recording ink, it is not yet attained to sufficiently satisfy both excellent image preservability against light, heat and high humidity and less blurring of image drawn, and realization thereof is demanded on the market.
[List of Patent Documents as Background Art]
The above-described documents as the background art are as follows.
[Patent Document 1]
JP-A-55-1.61856 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
[Patent Document 2] JP-A-61-36362
[Patent Document 3] JP-A-2-212566
[Patent Document 4]
JP-T-11-504958 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”)
[Patent Document 5]
Japanese Patent Application No. 2000-80733
An object of the present invention is to solve the problems of inkjet recording ink described above in the item of Background Art, more specifically, to provide an inkjet recording ink ensuring 1) excellent weather resistance (image preservability), 2) less blurring of image drawn and 3) high ejection stability.